I picked up a nice condition 1970 viking 6 hp outboard a couple weeks ago for my 12 footer. The first time I had it out it would run quite well at speed but idled very rough with lots of sputtering. I did some research on the motor and adjusted the premix and added seafoam and lead substitute, and had it out again for a night trolling. I was impressed by how well it idled, and trolled down beautifully all night, however my top end has dropped drastically (close to 50%). I consistently noticed that if I throttled up too fast it would struggle to find gas and sputter out, but if I go slowly it will eventually find the gas, but certainly not where it should be. Any idea as to what is causing this? I am guessing it is fuel delivery issue?